1. Field of the Invention
The present invention relates to a method for preparing the citraconic anhydride, and in particular relates to a method for preparing the citraconic anhydride from itaconic acid.
2. Description of the Related Art
Citraconic anhydride (also known as 2-methyl maleic anhydride) is a derivate of maleic anhydride. Citraconic anhydride is a product which has seen increasing demand by different industries. For example, citraconic anhydride can serve as an additive to improve the properties of unsaturated polyester resin or the raw material for preparing a diol compound via hydrogenation.
It is known in the art that citraconic anhydride can be prepared by pyrolysis and dehydration of citric acid (or a carboxylic acid containing six carbon atoms). In recent years, due to rapid advances in fermentation technology, itaconic acid can be conveniently prepared by microbiological fermentation of carbohydrates, such as glucose or glycerol. The obtained itaconic acid can serve as a raw material for preparing the citraconic anhydride via a dehydration/isomerization reaction.
In order to improve reaction yields, a catalyst in order to accelerate the isomerization reaction has been proposed. Thus, for example, U.S. Pat. No. 2,966,498 describes the manufacturing of citraconic anhydride from itaconic acid, using a catalyst based on alkali metal sulphate and dihydrogenophosphate.
U.S. Pat. No. 5,329,022 describes the manufacturing of citraconic anhydride from itaconic acid, using a catalyst based on amine or phosphine. The reaction additionally needs to employ xylene as a solvent and acetic anhydride to dehydrate water. An additional purification step is required, however, for removing the undesired acetic acid side product.
U.S. Pat. Nos. 5,670,659 and 5,824,820 describe the manufacturing of citraconic anhydride from itaconic acid in the presence of a catalyst, based on organic amine or organic acid salt, and an entrainer. However, since the organic catalyst is apt to react with itaconic acid to form oligomers, the reaction provides poor citraconic anhydride yields and it is difficult to separate the catalyst from the residual reaction solution.
WO No. 1995006026 describes the manufacturing of citraconic anhydride from itaconic acid in the presence of organic acids with sulfonic acid groups as a catalyst and a solvent as an entrainer. The organic acid catalyst, however, is apt to react with itaconic acid, resulting in low yields of less than 50%, and the organic acid catalyst is nonrecyclable.
WO 1994021589 describes the manufacturing of citraconic anhydride from itaconic acid in the presence of organic amide as a catalyst. The citraconic anhydride yield is less than 75%, and the organic amide catalyst is nonrecyclable
Therefore, it is necessary to develop a novel method for preparing the citraconic anhydride from itaconic acid with high yields. Accordingly, the disclosure is the provision of a process, for the preparation of citraconic anhydride, with high yield, from itaconic acid in the presence of a heteropolyacid or heteropolyacid salt as a catalyst, with or without a solvent.